Recently, a standard of an emission regulation of an evaporation that is a fuel leaked from an interior of a fuel tank to an exterior of the fuel tank becomes stricter and stricter. According to an Environmental Protection Agency (EPA) regulation and a California Air Resources Board (CARB) regulation, it is requested that a leakage of the evaporation from a slight opening of a fuel tank is sensed. According to JP2006-37752A, in an evaporation leakage checking system, a pressure of when a pump make an air flow through an orifice is sensed as a reference pressure, and then an existence of the evaporation leakage is checked based on the reference pressure and a pressure of when the pump increases or decreases a pressure in the fuel tank.